.TH std::copy_backward 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::copy_backward \- std::copy_backward

.SH Synopsis
   Defined in header <algorithm>
   template< class BidirIt1, class BidirIt2 >
   BidirIt2 copy_backward( BidirIt1 first, BidirIt1 last,       (constexpr since C++20)
   BidirIt2 d_last );

   Copies the elements from the range [first, last) to another range ending at d_last.
   The elements are copied in reverse order (the last element is copied first), but
   their relative order is preserved.

   The behavior is undefined if d_last is within (first, last]. std::copy must be used
   instead of std::copy_backward in that case.

.SH Parameters

   first, last      -      the range of the elements to copy from
   d_last           -      the end of the destination range
.SH Type requirements
   -
   BidirIt must meet the requirements of LegacyBidirectionalIterator.

.SH Return value

   Iterator to the last element copied.

.SH Complexity

   Exactly std::distance(first, last) assignments.

.SH Notes

   When copying overlapping ranges, std::copy is appropriate when copying to the left
   (beginning of the destination range is outside the source range) while
   std::copy_backward is appropriate when copying to the right (end of the destination
   range is outside the source range).

.SH Possible implementation

   template<class BidirIt1, class BidirIt2>
   BidirIt2 copy_backward(BidirIt1 first, BidirIt1 last, BidirIt2 d_last)
   {
       while (first != last)
           *(--d_last) = *(--last);
       return d_last;
   }

.SH Example


// Run this code

 #include <algorithm>
 #include <iostream>
 #include <numeric>
 #include <vector>

 int main()
 {
     std::vector<int> source(4);
     std::iota(source.begin(), source.end(), 1); // fills with 1, 2, 3, 4

     std::vector<int> destination(6);

     std::copy_backward(source.begin(), source.end(), destination.end());

     std::cout << "destination contains: ";
     for (auto i: destination)
         std::cout << i << ' ';
     std::cout << '\\n';
 }

.SH Output:

 destination contains: 0 0 1 2 3 4

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to            Behavior as published              Correct behavior
                       1. the behavior was well-defined if d_last
   LWG 1206 C++98      == last                                     1. made undefined
                       2. the behavior was undefined if d_last ==  2. made well-defined
                       first

.SH See also

   copy                  copies a range of elements to a new location
   copy_if               \fI(function template)\fP
   \fI(C++11)\fP
   ranges::copy_backward copies a range of elements in backwards order
   (C++20)               (niebloid)
